I believe you may have a difficult time finding a combination router and switch, or at minimum a Layer 3 switch, that affords you all the connectivity for that number of computers plus routing capabilities at an affordable level. You are going to have much better luck, features, and cost effective use from a business-class router and a separate switch.
For a lot of small business networks I use the Sonicwall series of firewalls because not only does it give you a fully configurable, VLAN capable, router designed to handle demanding business environments, but it also has all the firewall features for the best network security. Determining exactly which Sonicwall you would need may be a little difficult because we don't really know what kind of network traffic you do, network load and demand, and your connection speeds. A basic Sonicwal TZ 105 can be purchased for as little as $250, and have a vast range upwards to accommodate many different needs. Given the number of concurrent connections for the size of your network, you may be pushing the TZ 105 and might look into something closer to a 205 or 215 appliance.
Next, find a nice quality switch that not only offers you all the connectivity you need now, but can accommodate future growth. I've had great luck with the HP ProCurve line of switches, in particular the 1810 series offers a great feature set for low cost and again comes in a variety of configurations for port densities and speeds to fit your needs. A single 1810-24G would offer twenty-four gigabit ports for connecting up devices, but if you said you already are connecting up to two dozen computers, then this maxes out your switch already, and you may consider going with a 1810-48G to give you plenty of room to expand if you need.
Additionally, with the Sonicwall TZ series and the HP ProCurve 1810 series, you can build VLANs to help segment or separate network zones if you need to for added security. I don't know if this is something necessary for your business, but it's becoming more and more in demand as people's networks begin to grow. For instance, some small business we have rolled out new networks for recently have wanted to offer a public wireless network for customers or patrons - that should be in a separate VLAN. Perhaps you are running credit cards through your computers for transactions. Those computers handling any credit card transactions MUST be in a separate VLAN.